Previous studies have shown that MRI is a feasible imaging method for diagnosing deep neck infections in an emergency care setting [8] and that MRI has higher diagnostic accuracy for maxillofacial infections than CT [10]. We found that MRI has high diagnostic accuracy for odontogenic abscesses, that MRI findings can predict clinical severity and surgical approach and that MRI can point to the causative tooth. Together, these results add to the growing knowledge on the utility of emergency MRI in acute neck infections.
Accurate delineation of odontogenic abscesses is important for choosing the optimal type and extent of treatment (e.g., surgery). Sensitivity, specificity, and accuracy values of 0.95, 0.84, and 0.92 were found, respectively, for the MRI diagnosis of an odontogenic abscess. The high diagnostic accuracy is consistent with prior reports in larger samples of various types of neck infections [8]. The PPV of 0.95 is also markedly higher than that previously reported for CT (approximately 0.80) [13, 14]. All the false positives (four) were small abscesses (7–22 mm) and had imaging characteristics of an abscess, but no pus had been identified in surgery. However, small abscesses can also be missed in surgery, and pus can emerge between MRI and surgery. These issues may explain some of the false MRI findings in this study.
As expected, extensive infection findings (e.g., parapharyngeal or multi-space abscess) on MRI were associated with a more severe course of the disease (Supplementary Data Table 6). Previously, ME, RPE, and abscess diameter have been shown to predict more severe illnesses in deep neck infection patients with multiple etiologies [11]. Here, ME was a significant predictor of extraoral surgery in patients with odontogenic infections. SLS edema was more common here in odontogenic infections (77%) than was previously shown for tonsillar infections (24%) [12]. In contrast, RPE is less common in odontogenic than in tonsillar infections [11]. Thus, different etiologies of neck infection are associated with distinct soft tissue edema patterns, each with its own clinical significance.
Bone marrow signal changes were present in almost all cases. Based on recent consensus on the nomenclature for MRI of musculoskeletal infection outside the spine, these changes are consistent with acute osteomyelitis [15], although they do not necessarily indicate bone destruction, sequestration, or pus formation inside the bone, as is often associated with this term in the jaw [16]. In patients with suspected deep neck infection or abscess, these bone signal changes may be a valuable indicator of the odontogenic origin of infection because MRI with fat-saturated sequences is very sensitive to bone marrow edema. However, this analysis was restricted to patients already known to have an odontogenic infection, and the authors did not study the differentiation between odontogenic and non-odontogenic infections.
CT and CBCT imaging are considered reference standards for assessing dental emergencies, mostly due to their widespread availability and ability to depict bony structures [6, 7, 17, 18]. Although MRI is considered superior for evaluating soft tissues in deep neck infections [8, 9], whether it can also demonstrate the causative tooth in odontogenic infections has been unclear. Although there was variability among agreement between raters (kappa 0.66), it was found that MRI can pinpoint the causative infected tooth (within a margin of error of one tooth) with good accuracy. The fat-suppressed, contrast-enhanced, T1-weighted sequence was considered the most useful in this regard, and periapical enhancement is particularly suggestive of significant infection (Fig. 4). Gd is recommended for use in odontogenic infections, like in other soft-tissue or musculoskeletal infections [15]. A potential clinical implication of this study is that MRI alone may be sufficient to detect the odontogenic origin of severe neck infections needing medical imaging for suspected abscess, but lack of direct comparison with CT precludes strong practical recommendations. In addition, longer scanning times compared with X-ray or CT may be an issue in clinical practice when imaging the causative tooth.
Artifacts from dental hardware are a common limitation of CT in evaluating the oral cavity [19]. In general, MRI can also suffer from artifacts (such as distortion and signal loss), but these are considered less significant than those of CT in dental imaging [20]. In the subcohort, artifacts from dental hardware were rare and not considered significant in the decision-making.
The strengths of this study are its large sample size, high-quality 3 Tesla MR imaging with a Gd-based contrast agent and DWI, systematic neuroradiological evaluation of MRI findings, blinded multi-reader assessment thorough clinical characterization, and surgical confirmation of abscesses. However, as the study was retrospective in nature, the medical and surgical records may have been incomplete or imprecise. As indications for imaging may vary, the current results may be biased. Circumferential reasoning may also have biased the investigation. The MRI images might have had an impact on the final clinical decision regarding the causative tooth in this study. However, the authors did not consider this likely to significantly bias our results because, in clinical practice, MRI is not considered an established imaging method for indicating the causative tooth in odontogenic infections, and the exact tooth was not usually mentioned in the original reports. Interobserver agreement on the causative teeth was not perfect (Kappa of 0.66 indicates substantial agreement) but much improved when disagreement of one tooth was allowed. While this may indicate difficulties for radiologists in accurately numbering individual teeth (e.g., when some teeth are missing), MRI can pinpoint the region of the infected tooth with acceptable precision. A further limitation of this analysis is that only a subset of patients was included in whom the causative tooth was not known. The surgical results were also sometimes unclear, although in very few cases. The interpretation of some of the MRI findings may be subjective, although a substantial percent agreement was found for the bone marrow signal changes, as has also been found for soft tissue edema patterns [11, 12] and detection of abscesses [8]. The most important limitation is the lack of head-to-head comparisons between MRI and CT, although such data already exist in the literature [10]. The evidence seems to be accumulating that MRI can more accurately show the extent and origin of infection and abscess formation [8, 11, 12]. It should be recognized that MRI may not always be suitable or available for all patients, so these results may not apply to all facilities.
In conclusion, MRI provided clinically meaningful information in patients with odontogenic infections. These results add to our understanding of the clinical utility of MRI in acute neck infections.
